Are you trying to access an Ethernet and TCP/IP equipped PLC (probably on an isolated network) from a Boron, to allow control via the Particle cloud via cellular?
If so, there are two potential issues:
One is that the Ethernet FeatherWing does not support static IP addressing. So the Ethernet network must have a DHCP server, or it won’t bring up the TCP/IP layer.
The second is that once you have a valid link layer and accessible gateway on a Boron with an Ethernet FeatherWing, by default it assumes that it will be the valid connection to the Internet and will not attempt to connect to the cloud via cellular anymore. If there is no link (you unplug Ethernet) it will automatically switch to cellular, but if there is a link it will not fall back.
The second problem should be possible to work around in user firmware, but the DHCP requirement cannot be worked around without modifying Device OS.